FIG. 1 illustrates conventional workflows for acquiring images, rasterizing the acquired images into “digital contone” data, and manipulating the digital contone data into a format compatible to a printing device. In particular, FIG. 1 illustrates that various input sources 102, 104, 106 may be used to acquire an image by various image capture devices 108, 110, 112 and converted into a digital data file 114. For example, a digital camera 108 may take a digital picture of an analog contone scene 102 and convert the picture into a digital file 114. Examples of digital files 114 include a JPG file, a TIFF file, and any other digital file format known in the art. FIG. 1 also illustrates that a scanner 110 may be used to digitally scan a hardcopy print 104 and convert the digital scan into a digital file 114. Also, graphic design software 112 may be used to generate a graphic design 106 and to save such design as a digital file 114.
The digital file 114 includes a plurality of “pixels” arranged in a two-dimensional array. Each pixel includes intensity data associated with red, green, and blue color separations. However, the printing devices 116, 118, and 120 print images according to four different color separations cyan, magenta, yellow, and black, commonly denoted by CMYK, respectively. Accordingly, if a user desires to print the digital file 114 with any one of the printers 116, 118, 120, software, hardware, or both may be used as a Raster Image Processor “RIP” 122 to rasterize the digital file 114 into “digital contone” CMYK data 124. Specifically, the RIP 122 converts the digital red, green, and blue data in the digital file 114 into CMYK data 124.
In addition, the printers 116, 118, 120 typically have a much greater printing resolution than the image acquisition resolution of the devices 108, 110, 112. Accordingly, the RIP 122 typically increases the resolution of the image data it processes such that the digital contone CMYK data 124 has a greater resolution than the digital file 114. In other words, a “pixel” in the digital file 114 may correspond to several “RIPped pixels” in the digital contone CMYK data 124. A single RIPped pixel is illustrated with reference numeral 126.
In order to be printed, the digital contone CMYK data 124 is subjected to a halftone process 130 and converted to “ready-to-print” (“RTP”) data 128 that is compatible with the printing device that will print the RTP data. The RTP data 128 typically has the same or a greater resolution than the digital contone CMYK data 124. Accordingly, a RIPped pixel, such as RIPped pixel 126, typically corresponds to one or more elements of the RTP data 128, such elements being referred to herein as “exposure dots.” A single exposure dot is illustrated, for example, with reference numeral 138.
Depending upon the printer 116, 118, 120 being used and the type of image being printed, one of several halftone processes may be used, such as halftone processes 130. For example, if an operator wants to use the printer 116, the user may select the threshold halftone process 132 to convert the digital contone CMYK data 124 into the RTP data 134. In conventional threshold halftone processes, if an intensity of an input RIPped pixel 126 is greater than or equal to a threshold, then an exposure dot in the RTP data 134 corresponding to the RIPped pixel 126 is set to an ON value, indicating that an exposure dot is to be printed at that location. If the intensity value of the RIPped pixel 126 is lower than the threshold, then a corresponding exposure dot in the RTP data 134 is set to OFF, indicating that no exposure dot will be printed at that location.
If the user desires to print with printer 118, the user may select patterned dot halftoning 140 in order to generate the RTP data 142. According to patterned dot halftoning, depending upon the intensity value of the input RIPped pixel 126 and the relative resolutions of the printer 118 and the digital contone CMYK data 124, one of a plurality of patterns 144 will be used to generate a pattern of exposure dots in a halftone cell 146. In the example of FIG. 1, the halftone cell 146 corresponds to a RIPped pixel from the digital contone CMYK data 124 and comprises four exposure dots. In this case, the halftone cell 146 can represent five different intensity levels: (1) where all four exposure dots in the halftone cell 146 are “off”; (2) where one of the four exposure dots in the halftone cell 146 are “on,” and the rest are “off”; (3) where two of the four exposure dots in the halftone cell 146 are “on,” and the rest are “off”; (4) where three of the four exposure dots in the halftone cell 146 are “on,” and the other exposure dot is “off”; and (5) where all of the exposure dots in the halftone cell 146 are “on.” In this example, if the RIPped pixel being processed has an intensity value associated with little or no intensity, pattern (1) may be used for the corresponding halftone cell. If the RIPped pixel being processed has an intensity value associated with a higher level of intensity, pattern (2) may be used, and so on.
If a user desires to print the data 124 with a multilevel printer 120, the user may select the multilevel halftone process 148. A multilevel printer, as opposed to a binary printer, is able to print a single exposure dot having one of multiple intensities. For example, an 8-bit multilevel printer 120 can print any one exposure dot with one of 256 different exposure levels. In contrast, a binary printer can either print a single exposure dot with one of two intensity values: “on” or “off.” Accordingly, the multilevel halftone process 148 generates RTP data 150 with exposure dots 152 having one of a plurality of different exposure intensity levels, depending upon the capabilities of its associated multilevel printer. FIG. 2 illustrates exposure dots of a binary printer and FIG. 3 illustrates exposure dots of a multilevel printer. FIG. 4 illustrates a histogram of the digital contone CMYK data 124 and the resulting histogram of the RTP data 150 (also referred to as “multilevel halftone data”) after a multilevel halftone process 148 has been performed.
The halftone processes 130 are performed for each of the C, M, Y, and K color separations in the digital contone CMYK data 124. Accordingly, separate RTP data 128 is generated for and corresponds to each of the color separations C, M, Y, and K of the data 124. Further, the halftone processes use “screens,” which are essentially tables that are used to determine what RTP data should be output for the corresponding digital contone CMYK data 124. Typically, one screen is used for each color separation.
FIG. 5A illustrates a halftone screen 501 for a cyan color separation. The screen 501 has multiple “screen dots” 502 that represent locations where an exposure dot in the RTP data 504 will have a non-zero exposure intensity. In other words, screen dots 502 represent locations where a dot will be printed by a printing device. In order to generate the RTP data 504, the screen 501 is superposed, typically at an angle, on the digital contone data 505. Commonly, the halftone screen 501 is smaller (has a lower resolution) than the digital contone data 505 to which it is to be applied. Accordingly, the halftone screen 501 is tiled as it is superposed, typically at an angle, on the digital contone data 505, as shown at 506 in FIG. 5A. Each screen dot 502 translates the intensity value of the pixel it overlays into a corresponding exposure dot 503 having a particular exposure intensity value, as shown, for example, at 507 in FIG. 5A.
Conventionally, there have been two different types of halftone screens: AM screens and FM screens. An AM screen, shown, for example, at 510 in FIG. 5B, refers to an amplitude-modulated screen, which includes screen dots having a regular pattern. In contrast, an FM screen, shown, for example, at 511 in FIG. 5B, refers to a frequency-modulated screen, which exhibits screen dots having a random pattern. An FM screen also is referred to as a “stochastic screen.”
In order to produce pleasing images using AM screens, a set of AM screens are produced where each screen is configured for one of the CMYK color separations, and the screens are superposed on their corresponding digital contone data at particular angles. Typically, when the screens are superposed, the cyan screen is oriented at 15 degrees over its corresponding digital contone data, the magenta screen is oriented at 75 degrees, the black screen is oriented at 45 degrees, and the yellow screen is oriented at zero degrees. When each of these screens are overlayed at these specific angles, their screen dots produce a pleasing microstructure called a rosette structure that the human eye does not readily notice. However, interference patterns of screen dots called moiré patterns appear and occasionally degrade image quality when conventional AM screens are applied.
FM screens do not have the problems associated with the distracting moiré interference pattern. However, worm-like artifacts can be generated when using FM screens due to connections between screen dots in higher parts of the tone scale, i.e. parts of the tone scale where exposure intensity is high and screen dots are large and begin to join.
Further, although FM screens work well for high-resolution printing (approximately 5,000 or more dots per inch), such as that performed by high-resolution ink jet printers, they have been less effective for lower-resolution printing (approximately 2,000 or fewer dots per inch), such as electrophotographic, flexographic, direct imaging (“DI”), dye sublimation, and lower-resolution ink-jet printing devices. For example, electrophotographic (“EP”) printing and flexographic printing are not presently capable of printing at the resolutions offered by ink jet printing, because these methods of printing have a larger minimum exposure dot size than that of high-resolution ink jet printing. To elaborate, EP printing transfers toner to a printing substrate by adding spots of electric charge to an image cylinder, which attracts toner. The toner is then transferred to a substrate, such as paper. If the exposure dot size is too small, too small of a charge is added to the image cylinder to attract toner properly. Consequently, too little or no toner will be transferred to the substrate. In the case of flexographic printing, raised exposure dots are formed on a flexible printing plate. Ink is then applied to the flexible printing plate, and the raised exposure dots transfer the ink by contact to a substrate. If the raised exposure dots are too small on the printing plate, ink will not be properly transferred to the printing plate. Similar problems exist for other lower-resolution printing techniques. Because FM screens, however, offer advantages over AM screens, such as elimination of the moiré interference pattern, an FM screen that produces high quality images without artifacts for lower-resolution printing processes is desired.